DyslexiaA new model of this reading disorder emphasizes defects in the language-processing rather than the visual system. It explains why some very smart people have trouble learning to read.

by Sally E. Shaywitz, M.D.

One hundred years ago, in November 1896, a doctor in Sussex, England, published the first description of the learning disorder that would come to be known as developmental dyslexia. “Percy F., . . . aged 14,... has always been a bright and intelligent boy,” wrote W. Pringle Morgan in the British Medical Journal, “quick at games, and in no way inferior to others of his age. His great difficulty has been—and is now—his inability to learn to read.”

In that brief introduction, Morgan captured the paradox that has intrigued and frustrated scientists for a century since: the profound and persistent difficulties some very bright people face in learning to read. In 1996 as in 1896, reading ability is taken as a proxy for intelligence; most people assume that if someone is smart, motivated and schooled, he or she will learn to read. But the experience of millions of dyslexics like Percy F. has shown that assumption to be false. In dyslexia, the seemingly invariant relation between intelligence and reading ability breaks down.

Early explanations of dyslexia, put forth in the 1920s, held that defects in the visual system were to blame for the reversals of letters and words thought to typify dyslexic reading. Eye training was often prescribed to overcome these alleged visual defects. Subsequent research has shown, however, that children with dyslexia are not unusually prone to reversing letters or words and that the cognitive deficit responsible for the disorder is related to the language system. In particular, dyslexia reflects a deficiency in the processing of the distinctive linguistic units, called phonemes, that make up all spoken and written words. Current linguistic models of reading and dyslexia now provide an explanation of why some very intelligent people have trouble learning to read and performing other language-related tasks.

In the course of our work, my colleagues and I at the Yale Center for the Study of Learning and Attention have evaluated hundreds of children and scores of men and women for reading disabilities. Many are students and faculty at our university’s undergraduate, graduate and professional schools. One of these, a medical student named Gregory, came to see us after undergoing a series of problems in his first-year courses. He was quite discouraged.

Although he had been diagnosed as dyslexic in grade school, Gregory had also been placed in a program for gifted students. His native intelligence, together with extensive support and tutoring, had allowed him to graduate from high school with honors and gain admission to an Ivy League college. In college, Gregory had worked extremely hard and eventually received offers from several top medical schools. Now, however, he was beginning to doubt his own competence. He had no trouble comprehending the intricate relations among physiological systems or the complex mechanisms of disease; indeed, he excelled in those areas requiring reasoning skills. More problematic for him was the simple act of pronouncing long words or novel terms (such as labels used in anatomic descriptions); perhaps his least well-developed skill was rote memorization.

Both Gregory and his professors were perplexed by the inconsistencies in his performance. How could someone who understood difficult concepts so well have trouble with the smaller and simpler details? Could Gregory’s dyslexia--he was still a slow reader--account for his inability to name body parts and tissue types in the face of his excellent reasoning skills?

It could, I explained. Gregory’s history fit the clinical picture of dyslexia as it has been traditionally defined: an unexpected difficulty learning to read despite intelligence, motivation and education. Furthermore, I was able to reassure Gregory that scientists now understand the basic nature of dyslexia.

Over the past two decades, a coherent model of dyslexia has emerged that is based on phonological processing. The phonological model is consistent both with the clinical symptoms of dyslexia and with what neuroscientists know about brain organization and function. Investigators from many laboratories, including my colleagues and I at the Yale Center, have had the opportunity to test and refine this model through 10 years of cognitive and, more recently, neurobiological studies.